Field
The present invention relates to a backlighting apparatus for direct-view backlight units, particularly for LCD TVs, tablets, notebooks computers, and the like.
Discussion of the Background
Light emitting diodes (LEDs) have now become the preferred light sources for the backlight units (BLUs) of liquid crystal display devices (LCDs). As a consequence of their dramatic increase in efficacy, the number of LEDs required to adequately illuminate BLUs has decreased by a factor of 2 to 3 times over the past year. Also, consumer demand for increasingly thinner LCD TVs mean that there is a need for new optical architectures to allow for the manufacture of these thinner designs and yet still maintain adequate luminance and uniformity to the LCD display panels.
Previous optics for direct-view BLUs included small refractive lenses placed directly over LED light sources, arranged in geometrical arrays, and acting as baffles to redirect the intense light coming from the LEDs. As such, hot spots in the LCD panel were reduced to acceptable levels, with the result that the thickness of the resulting BLU was typically 20-25 mm. Consumer demand for thinner LCD TVs requires that a new optical architecture for BLUs be created to accommodate BLUs of 12 mm thickness or less. Thus, this patent application seeks to overcome the shortcomings of previous patents and applications for BLUs, such as disclosed in U.S. Patent Application Publication No. 2006/0138437A1, U.S. Pat. Nos. 7,348,723, 7,445,370, 7,621,657, 7,798,679, 8,227,969, and 8,328,395, and baffled LED elements such as disclosed in U.S. Pat. No. 6,007,209.
A typical flat-panel display may utilize an LCD to display information. LCDs, which allow the display of alphanumeric, graphic, or other information, comprise a medium whose transmittance changes in response to the application of electrical potential across the medium. LCDs may be viewed even in an otherwise dark environment, by applying illumination uniformly to their rear face. An exemplary backlighting apparatus for an LCD is disclosed in U.S. Pat. No. 4,043,636.
It may be desirable for a backlight for LCDs, such as those found in LCD TVs, to have certain attributes. Notebook computers and tablets are often used during travel and in areas with little light such as trains and airplanes. Hence, it may be desirable that LCDs have an acceptable level of brightness and uniformity so that the user can easily read the information on the display.
Furthermore, software applications that are used on notebook computers, tablets, and LCD TVs usually require color images. Hence the display may have spectral content that is compatible with the colors displayed on the LCD. It may also be desirable that the light source from the monitor be compact so that it will not significantly increase the size and weight of the notebook computer, tablet, or LCD TV. Many LCD TVs are currently made in a 16×9 format, and the thinness of the display may be a critical factor to its purchase by its end-user. Most LCD TV backlight units now require a total thickness of 25 mm or less, and most recently, less than 13 mm in thickness.
Previously, the ultra-thin backlights were constructed using a 3-4 mm thick acrylic sheet, with the LEDs being placed along one edge of the sheet, to couple the light from the LEDs into a plastic waveguide and conduct the light throughout the acrylic sheet using total internal reflection (TIR). The light was then ejected by using an out-coupling array of micro-dots or micro-lenses that disrupt the TIR and allow the light to be uniformly extracted to the back surface of the LCD display. The shortcoming of this method is that large acrylic sheets may be expensive and relatively heavy.
To make the backlight units less costly and lighter, manufacturers have turned to so-called direct view backlights. Here, the large acrylic sheet is replaced by an array of small, light, and inexpensive lenses that spread the light out over the surface of the backlight unit, such that it has sufficient illuminance and uniformity. However, as the thickness of the backlight unit decreases from 25 mm to the 11-12 mm range, the refractive optical principles upon which theses lenses are based may cease to yield acceptable uniformity. Thus, there is a need to create a new optical paradigm that will accommodate this 11-12 mm thick backlight requirement and still allow for the reductions in cost and weight, while maintaining good spatial uniformity.
U.S. Pat. No. 6,007,209 discloses baffling light coming directly from an LED to an LCD screen, such that LEDs could be arranged into a planar array of sources and then be effectively used as a BLU for an LCD display. At the time this patent issued, LEDs were not very efficient or powerful, but in the succeeding 15 years white LEDs have gone from an efficacy of 5 lumens/Watt, to more than 200 lumens/Watt. As a consequence, the baffling technology that shields the LED from creating a hot spot in the LCD display has to be correspondingly much better at hiding and distributing the light from the LED array so as to maintain excellent uniformity and an acceptable value of illuminance. Previous lens baffling elements such as those taught in U.S. Pat. Nos. 7,348,723, 7,445,370, 7,621,657, 7,798,679, 8,227,969, 8,328,395 must be improved upon to meet these new thinner BLU requirements.
To understand the shortcomings of the previous technologies in moving to these very thin BLU dimensions, we must examine a little recognized optical feature called Fresnel reflections. Over two hundred years ago, Augustin Fresnel showed that when light went from one material with a certain index of refraction to a second material with a different index of refraction, a certain fraction of the light beam would be transmitted and a certain part of the light beam would be reflected. Many direct-view BLUs employ baffles between the LED and LCD display screen that consists of an array of small refractive lenses, such as disclosed in U.S. Pat. No. 7,798,679.
As one moves to increasingly thinner BLUs, the light directed laterally may suffer Fresnel reflections from the lens/air interface, which are subsequently sent downward to the printed circuit board (PCB), which is may be white, and as a result retro-reflects that light directly onto the LCD screen causing non-uniformities (usually evidenced as bright circles from the rotational symmetry of the lens). Imprinting a faceted bottom surface has been shown to ameliorate this effect somewhat, but it increasingly becomes more of a problem for keeping good screen uniformity as the distance between the baffling lens and the surface of the LCD screen shrinks to distances of approximately 10 mm.